Area of a Quadrilateral

multiplying diagonal elements
Area = |(2*1-8*1)+(8*3-1*4)+(4*6-3*6)+(6*1-2*6)/2|=7
but the answer is 14 and it gives different answers for different choice of co-ordinate order
so how to judge which one is correct?
any limitation to use this rule?
my heads going crazy over this....

You're trying to use the fact that the area of a convex quadrilateral is the absolute value of half the cross-product of the diagonals? It's easier to compute the diagonals first, and make sure that the lengths you compute are really those of the diagonals

If you draw a lines between adjacent points in the order you've given you don't get a convex polygon.